1. Field of the Invention
This invention relates to a method and apparatus for transmitting light in a scattering medium.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in brackets [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Scattering of light by inhomogeneous media poses a fundamental challenge to numerous applications in astronomy, biomedical imaging and colloidal optics. For a long time, scattered light has been viewed as a source of noise. Many advanced imaging approaches have been developed to filter it out, relying solely on the ballistic light component. However, this strategy is futile in strong scatterers, such as optical diffusers, paint or thick layers of biological tissue, which do not transmit a ballistic component. Focusing beyond the ballistic regime has therefore long been considered impossible.
Recent developments in the field of wavefront shaping have changed this view [1], demonstrating that scattered light can be utilized for optical focusing beyond the ballistic regime. As light travels across a strong scatterer, the wavefront leaving the sample is seemingly randomized. But, in fact, there is a linear mapping between the optical modes in the input wavefront and the optical modes in the output wavefront, which can be fully described by a scattering transmission matrix. These linear, deterministic and time-symmetric properties of scattering [2] have been harnessed for focusing and image transfer across complex samples by iterative wavefront optimization [3-8], time reversal [9,10], or directly measuring and inverting the transmission matrix [11-13].
Despite these significant advances in our understanding of wavefront control across scatterers, the methods outlined above require direct access to both sides of the scatterer (i.e. the input plane and the target plane). These approaches are therefore not directly applicable when the goal is to focus between scatterers or deep inside a scatterer. In such cases, wavefront optimization requires the assistance of beacons or so-called “guide-stars” in the target plane. Guide-stars have successfully been implemented using second-harmonic [14] or fluorescent [15] particles, but optical focusing inside scattering samples is limited to the vicinity of these stationary particles. An alternative approach, termed time reversal of ultrasound-encoded light (TRUE) [16-20], shows much promise for non-invasive imaging by taking advantage of virtual acousto-optic beacons. In this approach, an ultrasound focus frequency-shifts the scattered optical wavefront within a scattering sample, thus creating a source of frequency-shifted light. Scattered, frequency-shifted light emanating from this source is recorded outside the tissue and time-reversed by optical phase-conjugation to converge back onto the location of the ultrasound focus. Despite its ability to focus inside scattering samples at unprecedented depths, the resolution of TRUE imaging is fundamentally limited by the size of the ultrasound beacon, which is at least an order of magnitude larger (tens of micrometers at best) than the optical speckle size (micrometer-scale).
Here, the present invention proposes a way to break this resolution barrier imposed by the size of the beacon by time-reversal of variance-encoded light (TROVE). TROVE takes advantage of the spatially unique variance imposed by the acoustic field to encode the spatial location of individual optical speckles within the ultrasound focus. Upon optical time reversal of computationally decoded modes, one or more embodiments achieve focusing at the scale of single optical speckles with diffuse light.